An Afghan police officer stands guard during a campaign rally in the Paghman district of Kabul, Afghanistan, Monday, June 9, 2014. The second round of Afghanistan's presidential election will take place on June 14, 2014. (AP Photo/Rahmat Gul)

White House principal deputy press secretary Josh Earnest speaks to the media during the daily news briefing at the White House in Washington, Tuesday, June 10, 2014. Earnest answered questions including on the recent soldier deaths in Afghanistan. (AP Photo/Jacquelyn Martin)

KABUL, Afghanistan (AP) — Five American special operations troops were killed by a U.S. airstrike called in to help them after they were ambushed by the Taliban in southern Afghanistan, in one of the deadliest friendly fire incidents in nearly 14 years of war, officials said Tuesday.

The deaths were a fresh reminder that the conflict is nowhere near over for some U.S. troops, who will keep fighting for at least two more years.

Pentagon spokesman Rear Adm. John Kirby said the five American troops were killed Monday "during a security operation in southern Afghanistan."

"Investigators are looking into the likelihood that friendly fire was the cause. Our thoughts and prayers are with the families of these fallen," Kirby said in a statement.

In Washington, two U.S. defense officials said the five Americans were special operations force members, but they were not more specific. The officials spoke on condition of anonymity because notification of the families of the five had not yet been completed.

The deaths occurred during a joint operation of Afghan and NATO forces in the Arghandab district of southern Zabul province ahead of Saturday's presidential runoff election, said provincial police chief Gen. Ghulam Sakhi Rooghlawanay. After the operation was over, the troops came under attack from the Taliban and called in air support, he said.

"Unfortunately five NATO soldiers and one Afghan army officer were killed mistakenly by NATO airstrike," Rooghlawanay said.

There was no way to independently confirm Rooghlawanay's comments. The coalition would not comment and NATO headquarters in Brussels also declined to comment.

However, special operations forces often come under fire on joint operations and are responsible for calling in air support when needed. Because of constraints placed by President Hamid Karzai, such airstrikes are usually called "in extremis," when troops fear they are about to be killed.

Airstrikes have long caused tensions between the Afghan government and coalition forces, especially when they cause civilian casualties. Airstrikes that kill coalition soldiers are far less common. One of the worst such incidents came in April 2002 when four Canadian soldiers were killed by an American F-16 jet fighter that dropped a bomb on a group of troops during night firing exercise in southern Kandahar.

The Taliban claimed responsibility for Monday's ambush in Zabul.

A Taliban spokesman, Qari Yousef Ahmadi, said a battle took place between foreign troops and Taliban fighters in the Arghandab district, and a "huge number" of NATO soldiers were killed or wounded in the fighting. The Taliban often exaggerate their claims.

The insurgents have intensified attacks on Afghan and foreign forces ahead of Saturday's presidential runoff, and officials are concerned there could be more violence around the time of the vote, although the first round in April passed relatively peacefully.

Of the 30,000 or so U.S. troops left in Afghanistan, special operations forces are among the only ones that are active on the battlefield, mentoring and advising Afghan commandos during raids.

An even smaller group that operates independently of the NATO coalition mandate, which expires at the end of the year, goes after high-value targets including the remnants of al-Qaida. Many of those special forces are likely to remain after the end of 2014, when foreign combat troops leave the country.

Although the U.S. has pledged 9,800 troops will remain until the end of 2016, a bilateral security agreement allowing them to do so has yet to be signed. The two candidates vying to succeed Karzai have said they will sign the deal.

Most of those troops will be training and advising the Afghan army and police, but a small counterterrorism force will still go after high value Jihadists still in the country.

The main opposition candidate, former foreign minister Abdullah Abdullah, has little love for the Taliban and is unlikely to stand in the way of such operations. The other contender, former finance minister and Karzai adviser Ashraf Ghani Ahmadzai, may be more reticent.

Separately, a NATO statement said a service member died Monday as a result of a non-battle injury in eastern Afghanistan.

The deaths bring to 36 the number of NATO soldiers killed so far this year in Afghanistan, with eight service members killed in June.

Casualties have been falling in the U.S.-led military coalition as its forces pull back to allow the Afghan army and police to fight the Taliban insurgency. All combat troops are scheduled to be withdrawn from the country by the end of this year.

In eastern Ghazni province, insurgents kidnapped 33 university instructors who were travelling to Kabul for a seminar. Kandahar provincial spokesman Dawa Khan Menapal said the 33 were taken by a large group of insurgents and there was no word on their fate. He said all were from the southern province.

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